This invention relates to surge voltage arresters of the type primarily used for protecting telephone and like communication lines from overvoltage or overcurrent surges. The principles of the present invention are applicable to station protectors as well as to central office protectors. A typical central office protector is shown in U.S. Pat. No. 3,818,271, granted June 18, 1974 to Baumbach.
The use of varistors of the zinc oxide compound type in surge voltage arresters is broadly known as by reference to U.S. Pat. No. 4,092,694 to Stetson, granted May 30, 1978. Varistors of this type are non-linear voltage dependent resistances in which the resistance decreases as increasing voltage is applied across the varistor. Such varistors are also sensitive to heating. With increasing temperatures, the leakage current across the varistor increases at a given voltage. The increased leakage current further raises the temperature with the result that at a critical temperature and voltage condition, the varistor becomes subject to a thermal runaway condition and fails due to passing ever-increasing current. Upon failure generally a hole will develop in the varistor and hot particles will be expelled. Such a condition is obviously unsuitable for use in a central office line protector or in the proximity of other equipment because damage to such equipment and to personnel becomes possible.
The thermal runaway condition can be opposed by improving the heat transfer between the varistor and its supporting housing. However, the use of some form of improved thermal coupling may not be adequate to prevent thermal runaway under high surge voltage conditions of long duration, particularly if the surge voltage arrester is to be produced at reasonable costs.